Such sensor elements are used especially in motor vehicles for measuring air/fuel gas mixture compositions. More specifically, sensor elements of this type are used as so-called “lambda probes”, and they play an important part in the reduction of pollutants in exhaust gases, both in Otto engines and in Diesel technology.
In combustion technology, the so-called air ratio “lambda” (λ) generally denotes the ratio of an actually supplied air mass to an air mass theoretically required (i.e., stoichiometric air mass). The air ratio is measured using one or more sensor elements mostly at one or more locations in the exhaust tract of an internal combustion engine. Accordingly, “rich” gas mixtures (i.e. gas mixtures having an excess in fuel) have an air ratio λ<1, whereas “lean” gas mixtures (i.e. gas mixtures having a fuel deficiency) have an air ratio λ>1. Besides the motor vehicle technology, such and similar sensor elements are also used in other fields of technology (especially in combustion technology), e.g., in aviation technology or in the control of burners, for instance, in heating systems or power plants. Numerous different developments of the sensor elements are known from the related art and described, for instance, in Robert Bosch GmbH: “Sensoren im Kraftfahrzeug” [Sensors in the Motor Vehicle], June 2001, p. 112-117, or in T. Baunach et al.: “Sauberes Abgas durch Keramiksensoren” [Clean Exhaust Gas Through Ceramic Sensors], Physik Journal 5 (2006) No. 5, p. 33-38.
One development is the so-called voltage-jump sensor, whose measuring principle is based on measuring an electrochemical potential difference between a reference electrode exposed to a reference gas and a measuring electrode exposed to the gas mixture to be measured. Reference electrode and measuring electrode are interconnected via the solid state electrolyte; because of its oxygen-ion conducting properties, zirconium dioxide (i.e., yttrium-stabilized zirconium dioxide) or similar ceramics are normally used as solid state electrolyte. Theoretically, the potential difference between the electrodes, especially in the transition between rich gas mixture and lean gas mixture, exhibits a characteristic abrupt change, which can be utilized to measure and/or control the gas mixture composition. Different exemplary embodiments of such voltage jump sensors, which are also known as Nernst cells, are discussed in DE 10 2004 035 826 A1, DE 199 38 416 A1 and DE 10 2005 027 225 A1, for example.
Alternatively or in addition to voltage-jump sensors, so-called “pump cells” are used as well, in which an electrical “pump voltage” is applied to two electrodes connected via the solid state electrolyte, and the “pump current” is measured by the pump cell. In contrast to the principle of the voltage-jump sensors, in the case of pump cells both electrodes are usually in contact with the gas mixture to be measured. In the process, one of the two electrodes is directly exposed to the gas mixture to be measured (usually via a permeable protective layer). As an alternative, this electrode may also be exposed to an air reference. However, the second of the two electrodes is usually implemented in such a way that the gas mixture is unable to reach this electrode directly, but instead must first pass through a so-called “diffusion barrier” in order to reach a cavity adjoining this second electrode. In most cases, a porous ceramic structure having selectively adjustable pore radii is used as diffusion barrier. If lean exhaust gas penetrates this diffusion barrier and enters the cavity, then oxygen molecules are electrochemically reduced to oxygen ions at the second, negative electrode by the pump voltage, transported through the solid state electrolyte to the first, positive electrode, where they are released again in the form of free oxygen. The sensor elements are mostly operated in what is referred to as limit current operation, which means in an operation in which the pump voltage is selected in such a way that the oxygen entering through the diffusion barrier is pumped completely to the counter electrode. In this operation the pump current is approximately proportional to the partial pressure of the oxygen in the exhaust-gas mixture, which is why sensor elements of this type are frequently also referred to as proportional sensors. In contrast to voltage-jump sensors, pump cells are able to be used for the air ratio Lambda across a relatively wide range, so that pump cells are used especially in so-called broadband sensors, for measuring and/or controlling also in the presence of gas mixture compositions beyond λ=1.
The afore-described sensor principles for voltage-jump cells and pump cells may advantageously also be used in combination, in what is known as “multicellulars”. For instance, the sensor elements may include one or more cells operating according to the voltage-jump sensor principle, and one or more pump cells. One example of a so-called double-cellular is discussed in EP 0 678 740 B1. Using a Nernst cell, the partial oxygen pressure in the afore-described cavity, adjoining the second electrode, of a pump cell is measured, and the pump voltage is corrected by a closed-loop control in such a way that the condition λ=1 prevails in the cavity at all times. Different modifications of this multi-cellular design are known.
However, the sensor elements known from the related art having a single-cell system in which two electrodes are exposed to the exhaust gas or the measuring gas frequently show a definite characteristic curve only with lean exhaust gas. In slightly lean operation, however, i.e., when λ approaches the value of 1, a deviation of the pump current characteristic curve from the theoretical curve is frequently noticed. Instead of a drop in the pump current at Lambda values that decrease in the direction of the value λ=1, a rise in the pump current may often even be seen. Because of this deviation the pump current curve no longer exhibits an unambiguous characteristic from which the air ratio is inferable. This has a negative effect on Lambda sensors to be used in Diesel vehicles, for example, which are operated in slightly lean operation in general.